1. Field of the Invention
The present invention relates to an equalizer circuit for communication signals, which is designed in the manner of an active Bode equalizer employing an amplifier having anti-phase outputs and an output-connected bridge circuit.
2. Description of the Prior Art
Variable equalizer circuits have been disclosed by H. W. Bode in the periodical "Bell System Technical Journal" 1938, Vol. 17, pp. 229-244, for which the expression "Bode equalizer" has also become a standard. As is known, these are circuits in which, fundamentally, a bridged T-section is employed. Additional circuits are assigned to the bridge arm and to the shunt arm which are also referenced as auxiliary quadripoles with proper wave impedance. The manner of operation and the use of possibilities of such equalizers are known per se, so that they need not be discussed herein in detail, the above citation being fully incorporated herein by this reference. The transfer function T.sub.r (j.omega.) of such equalizers can generally be presented as ##EQU1## where T.sub.0.sup. -1 =q.sub.0 =e.sup.a 0 is the residual attenuation factor, .theta. is a real switching constant with .vertline..theta..vertline..ltoreq.1, H.sub.0 is a circuit-dependent shift constant, and H (j.omega.) is a function determining the frequency response of the transfer function with .vertline.H (j.omega.).vertline..ltoreq.1.
The equalizers mentioned above are realized with concentrator elements, i.e. with resistors, coils and capacitors. If one wishes to realize attenuation equalizers having high attenuation shifts for .vertline..theta..vertline.&lt;1 with such structures, then the residual attenuation necessary for a predetermined attenuation shift increases rapidly, which again requires the use of additional amplifier circuits.
Because of the desire to provide at least partial alleviation, active equalizer circuits have also been heretofore disclosed. An equalizer which, in a certain sense, can at least be designated as an active Bode equalizer is specified in the periodical IEEE Transactions on Circuits and Systems, Vol. CAS-22, No. 5, May 1975, pp. 415-418, also fully incorporated herein by this reference. The difficulty occurring in this known equalizer can, above all, be seen therein that an excursion in both directions is possible only with the assistance of a variable negative resistance. This limits the areas of application to frequencies which lie below 10 kHz.
An equalizer of the type initially mentioned is specified for frequencies which lie below 10 MHz in the publication IEEE Transactions on Circuits and Systems, Vol. CAS-24, No. 6, June 1977, pp. 318-320, also fully incorporated herein by this reference. In this equalizer the transfer function T.sub.r of expression (1) above is realized in the form ##EQU2## by employing two operational amplifiers having differential inputs and differential outputs with the assistance of inverse feedbacks. The known circuit has the characteristic that the maximum shift is limited to .+-.9 dB, whereby the maximum shift is only achieved with Z (j.omega.) as a reactance network.
The residual attenuation is not freely selectable; independently of the shift, it amounts to 6 dB. The switching constant is, for example, .vertline..theta..vertline..ltoreq.0.5. What further complicates the practical realization of this known circuit is that two reactance bipoles must be designed so as to be completely identical to one another. For this reason, the circuit is relatively sensitive to manufacturing tolerances, since deviations from the rated values of the circuit elements already have a significant disruptive effect on the symmetry of the circuit.